Proteomic phenotyping of stimulated Müller cells uncovers profound pro-inflammatory signaling and antigen-presenting capacity

Abstract

Mueller cells are the main macroglial cells of the retina exerting a wealth of functions to maintain retinal homoeostasis. Upon pathological changes in the retina, they become gliotic with both protective and detrimental consequences. Accumulating data also provide evidence for a pivotal role of Mueller cells in the pathogenesis of diabetic retinopathy (DR). While microglial cells, the resident immune cells of the retina are considered as main players in inflammatory processes associated with DR, the implication of activated Mueller cells in chronic retinal inflammation remains to be elucidated. In order to assess the signaling capacity of Mueller cells and their role in retinal inflammation, we performed in-depth proteomic analysis of Mueller cell proteomes and secretomes after stimulation with INFγ, TNFα, IL-4, IL-6, IL-10, TGFβ1, TGFβ2 and TGFβ3. We used both, primary porcine Mueller cells and the human Mueller cell line MIO-M1 for our hypothesis generating approach. Our results point towards an intense signaling capacity of Mueller cells, which reacted in a highly discriminating manner upon treatment with different cytokines. Stimulation of Mueller cells results in a primarily pro-inflammatory phenotype with secretion of cytokines and components of the complement system. Furthermore, we observed evidence for mitochondrial dysfunction, implying oxidative stress after treatment with the various cytokines. Finally, both MIO-M1 cells and primary porcine Mueller cells showed several characteristics of atypical antigen-presenting cells, as they are capable of inducing MHC class I and MHC class II with co-stimulatory molecules. In line with this, they express proteins associated with formation and maturation of phagosomes. Thus, our findings underline the importance of Mueller cell signaling in the inflamed retina, indicating an active role in chronic retinal inflammation underlying the pathogenesis of diabetic retinopathy.